Patent Document 1 listed below discloses a phase angle varying apparatus (referred to as phase varying apparatus 1) for varying the relative phase angle between the crankshaft and the camshaft to change valve timing, the apparatus 1 equipped with a self-locking mechanism for preventing an undesirable change in phase angle caused by an external disturbing torque. The phase varying apparatus 1 is adapted to advance the phase angle of the camshaft relative to the crankshaft (not shown) in the phase advancing direction D1 or in the phase retarding direction D2 so as to recover the valve timing as required. This is done by enabling either a first electromagnetic clutch first electromagnetic clutch 21 or a second electromagnetic clutch 38 as shown in FIG. 1 of the Patent Document 1.
In the phase varying apparatus 1, the camshaft (not shown) has a center shaft 7 integral therewith, which rotatable supports a drive rotor 2 driven by the crankshaft as shown in, for example, FIG. 1. The center shaft 7 has a circular eccentric cam 12. The center shaft 7 is integrated, via a lock plate bush 13, with a lock plate 14 that holds the circular eccentric cam 12 ad with a first control rotor 3 by means of a coupling pin 2a. The camshaft (not shown) rotating together with a drive rotor 2 is in rotation in the D1 direction together with the drive rotor 2 when the first electromagnetic clutch 21 is not activated. When, however, the first control rotor 3 is retarded by the braking action of the first electromagnetic clutch 21, the camshaft is retarded relative to the drive rotor 2, thereby changing the phase angle of the camshaft relative to the drive rotor 2 in the D2 direction. On the other hand, the drive rotor 2 is integrated with a pin guide plate 33 via a first link pin 34. When the second electromagnetic clutch 38 puts a brake on a second control rotor 32, the first link pin 34 is displaced in a first radially shrinking guide groove 31 of the first control rotor 3 and a guide groove 33b extending in a substantially radial direction (hereinafter referred to radial guide groove 33b) of the pin guide plate 33, thereby rotating the camshaft in the D1 direction relative to the drive rotor 3. Consequently, the phase angle of the camshaft relative to the drive rotor is advanced in the D1 direction.
On the other hand, the phase varying apparatus of Patent Document 1 is equipped with a self-locking mechanism 11 for preventing the a change in phase angle between the camshaft and the drive rotor 2 from occurring by unrotatably fixing the lock plate 14 to the drive rotor 2. This locking of the lock plate is enabled by taking advantage of the external disturbing torque inputted to the camshaft which otherwise results in an angular displacement of the camshaft. Describing the self-locking mechanism 11 in more detail, the drive rotor 2 is a body comprising a sprocket 4 and a drive cylinder 5 integrated together and the lock plate 14 is inscribed in the inner periphery 20a of the cylinder section 20 of a drive cylinder 5. In what follows the line passing through the central axis L0 of the camshaft (the axis referred to as camshaft axis L0) and the cam center L1 of the circular eccentric cam 12 will be referred to as line L2, the line crossing the line L2 at the cam center L1 at a right angle referred to as line L3, the points at which the line L3 intersects the inner periphery 20a referred to as P3 and P4, as shown in FIG. 7 of the Patent Document 1. The angles made by the line L4 tangent to the lock plate 68 at the points P3 and P4 and the line L5 perpendicular to the line L3 will be referred to as θ1 and θ2, respectively. The coefficient of friction between the inner periphery 20a and the periphery of the lock plate 14 will be referred to as μ.
When the camshaft is subjected to an external disturbing torque that has arisen from reaction of a valve (not shown) and forces the camshaft to rotate in the D2 or the D1 direction, the cam center L1 of the circular eccentric cam 12 is forced to rotate about the camshaft axis L0, generating radially outward forces F1 and F2 acting on the inner periphery 20a in contact with the lock plate 14 at the points P3 and P4.
Under this condition, the tangential components of the three F1 and F2 acting on the periphery of the lock plate 14 are F1*sin θ1 and F2*sin θ2 respectively, urge the lock plate 14 to rotate within the cylinder section 20. On the other hand, the normal components of the forces F1*sin θ1 and F2*sin θ2, respectively, press the lock plate 14 onto the inner periphery 20a and generate frictional forces, μ*F1*sin θ1 and μ*F2*sin θ2 respectively, in the direction opposite to the tangential forces. When the tangential forces exceed the opposing frictional forces, the camshaft is rotated together with the locked lock plate 14 relative to the drive rotor 2, thereby rendering the camshaft out of phase relative to the crankshaft.
In view of the above-mentioned problem, the self-locking mechanism 11 of the Patent Document 1 is configured such that, when an external disturbing torque is transmitted to the crankshaft, the frictional forces overcome the tangential components to stop the rotation of the lock plate 14 and prevent an undesirable gap in phase angle from occurring between the camshaft and the crankshaft. Specifically, since the lock plate 14 is unrotatably fixed to the drive rotor 2 as a result of the self-locking effect when the following conditionsμ*F1*cos θ1>F1*sin θ1 and F2*sin θ2>μ*F2*cos θ2,are met, the angles θ1 and θ2 of the phase varying apparatus of the Patent Document 1 are set such thatθ1<tan−1μ and θ2<tan−1μ